Radio receiver



R. RECHNITZER RADIO RECEIVER Filed Feb. 26, 1935 April-1 3, 1937 INVENTDR. V R. RECHNITZER 7%4MM ATTQRNEY.

Patented Apr. 13, 1937.

RADIO. RECEIVER Rudolf Rechnitzer, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telej graphic n. b. H., Berlin, Germany, a corporation of Germany Application February 26, 1935, Serial No. 8,251

- In Germany March 12, 1934 Claims. "(01. 250-20) An object of this invention is to provide a novel circuit arrangement for eliminating an undesired frequency but which at the same time amplifies the desired signal frequency.

A further object of the invention is to provide angamplifier with an input circuit a portion of which is also common to its output'circuit and which has a resonant frequency for the signal andis also resonant at a second frequency correspending to the undesired frequency, which in the'case' of a superheterodyne receiver, may be the image frequency.

-For a-betterunderstanding of the invention reference is made to the accompanying specifica- 5 tion and drawing, in which:

Fig. l is a schematic circuit diagram of a high frequency amplifier arrangement,

Figs. 2 and 3 are schematic circuit diagrams of amplifiers arranged according to my invention 20 and,

Fig. 4 is a schematic circuit diagram of a combined oscillation generator and detector embodying the invention.

For a number of practical applications in re- 25 ceiving and transmitting it is desirable to make available circuit arrangements which, inside a certain frequency range. are at the same time capable of current and voltage resonance between two points, the distance in terms of fre- 30 quency between these two resonances being as far as feasible constant within the desired frequency range.

A problem of the said kind, for instance, arises in the case. of superheterodyne receivers designed 5 to suppress the image frequency. It is known from practice that undesirable disturbances frequently are occasioned due to the circumstance that an intermediate frequency is formed not only between the transmitter to be received and 40 the local oscillator, but also between the latter and a station frequency different from the station to be received an amount equal to twice the intermediate frequency. According to this invention, between the points between which our- 45 rent and voltage resonance is to prevail at the same time,say, between grid and cathode of an electron tube, a series system substantially'comprising an inductance coil, a stationary (fixed) condenser and a parallel circuit consisting of an 50 inductance and a rotary condenser are provided.

Fig. 1 shows a normal circuit arrangement for receiving. I denotes the electron tube to the grid of which is connected a parallel arrangement consisting of a coil L and a condenser C, while 55 the other end thereof is grounded. Between latory circuit comprising substantially L and 0,

ground and cathode is interposed a resistance R which is bridged or shunted for radio frequency by means of a blocking condenser C'.. The said resistance R is connected, for instance, at the same time in the plate circuit of a detector tube, so that it will be traversed by plate currents of varying values. The fall of potential occasioned across the same due to the plate current serves for the production of the requisite grid biasing voltage for tube i, optionally for the variation of 10 the grid biasing potentialwith a view to insuring automatic fading compensation or regulation. The signal frequency may be impressed on the grid circuit by a primary transformer winding 5.

Fig. 2 shows'an improved scheme compared with the circuit organization in'Fig. 1. In this scheme a choke-coil L is connected in series with the resistance R. For the undesired resonance frequency or image frequency of the oscilgrid and cathode are short-circuited. The plate current in this case passes through L as well as R, and condenser C has a larger value than the maximum value of. C so as to have negligible effect on the resonant frequency.

In order to insure synchronism, as it were, be-

. tween the two resonances, to be more precise,

in order to make conditions so that on changing the tuning to the signal frequency of the flywheel circuit consisting of L and C the tuning of the short-circuit consisting of L and C may stay at the same distance, in terms of frequency, from the signal frequency to which the parallel tuned circuit LC is adjusted, a condenser C is connected as shown in Fig. 3. The size of the same is readily obtainable by calculation and will depend on the signal and image frequencies involved. By choosing this condenser of proper value, the difference between the image frequency to which the'circuit comprising the series con- 40 nected elements C C L' is resonant and the frequency to which circuit LC is resonant, will remain substantially constant as condenser C is adjusted. The said condenser could be inserted also in the lead between the grid of tube I and the fiy-wheel circuit, L, C. In order to preclude charging up of the grid, it is necessary to provide a leak resistance R.

Fig. 4 shows a complete circuit organization in which the additional circuit elements are containedat the same time in other circuits and fulfill besides other functions and purposes. The choke-coil L- serves as a tickler in order that the tube conjointly' with the oscillator circuit 0 may be caused to start oscillating spontaneously at the frequency to which is tuned by condenser 6. Z is the output circuit tuned to the intermediate frequency. Inasmuch as it has been ascertained by tests that the size of the blocking condenser C serving to shunt the resistance R may be varied within wide limits without its effect being incidentally impaired, the condensers C and C of Fig. 3 are replaced by a single condenser K which then causes the image frequency suppressed to change as condenser C is adjusted to the signal frequency.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. In a superheterodyne radio receiver subject to interference from undesired signal currents of the image frequency, the combination of a vacuum tube having a cathode, grid and plate, a resonant input circuit comprising a coil and a condenser connected in parallel connected between said grid and cathode, a plate circuit connected to said plate and a circuit common to said input and plate circuits said last named circuit comprising a coil whose inductance has a value such as to form with the effective capacity of said resonant input circuit a circuit resonant at the undesired image frequency.

2. A superheterodyne radio receiver subject to interference from undesired signal currents of the image frequency and comprising the combination of a vacuum tube having a cathode, grid and plate, a circuit connecting said grid and cathode including the series connection of a first coil, a condenser and a second coil shunted by a second condenser, said first coil having such inductance value as to resonate with the effective value of said second condenser at the undesired image frequency and an output circuit connected to said plate.

3. A combined oscillator and detector subject to interference from undesired signal currents of the image frequency, the combination of a vacuum tube having a cathode, a grid and a plate, an input circuit connecting said cathode and grid comprising a coil shunted by a variable condenser, an output circuit connected to said plate and a feedback connection between said output and input circuits comprising an inductance coil common to said input and output circuits said inductance coil forming with the effective capacitance of said input circuit a circuit resonant at the undesired image frequency.

4. A superheterodyne radio receiver subject to interference from undesired signal currents of the image frequency and comprising the combination of a vacuum tube having a cathode, grid and plate, an input circuit connecting said grid and cathode comprising a coil and condenser connected in parallel to form a circuit resonant to a desired signal frequency, an output circuit connected to said plate and a resistor and second coil in series common to said input and output circuits, said second coil forming with said condenser a path of low impedance to currents of the undesired image frequency in said input circuit. 5. The combination defined in the preceding claim in which a condenser having a low impedance to currents of the signal and undesired frequencies is shunted across said resistor.

RUDOLF RECHNITZER. 

